FIELD OF THE INVENTION This invention relates to a method and system for driving an electromagnetic device and, more particularly to a method and system for controlling an electric current in the device.

BACKGROUND OF THE INVENTION Generally, electromagnetic devices to which the invention pertains have a stator core on which an operation coil is wound, and a movable core (hereinafter referred to as an armature) faced with the stator core via a gap and coupled to a load to be driven. Various examples of such devices are known, e. g., electromagnets, electric motors, actuators, relays, clutches, etc. When the stator core is energized, the movable core is attracted by a magnetic force and travels relative to the stator core. During this movement, the movable core moves against the force caused by the load and may produce useful operations, such as switching an electromagnet, spinning an electromagnetic motor, etc.

Electromagnets are used to lift heavy masses of magnetic materials and to attract movable magnetic parts of electric devices. For example, they may be employed in actuators for opening and closing a loaded valve. Typically, the operation coil in actuators is energized by a driving rectangular wave voltage until the armature fully reaches its one operating position (e. g., open position) and a desired period of time thereafter while the armature is held by the electromagnet. When the coil is de-energized, the armature may move towards and into another operating position (e. g., closed position) usually by a biasing mechanism, e. g., a spring.

Referring to Fig. 1, a simplified electric circuit 1 of a typical electromagnet is illustrated. The circuit comprises a DC power supply voltage U, a resistance R, and an inductance L. When the switch S is closed, the voltage U is suddenly applied to the series circuit 1. According to Kirchhoff's second law, the differential equation for the current I in the circuit 1 is given by U = RI + d/dt(LI) (1) Eq. (1) comprises an active term RI and a reactive term - d/dt(LI), and describes the evolution of the current I in time.

Referring to Fig. 2, a schematic diagram illustrating waveforms of the conventional electromagnet apparatus is illustrated. When the power supply voltage (U in Fig. 1) is applied at time to the coil current I starts to increase. At this stage, the inductance (L in Fig. 1) of the electromagnet is almost constant and Eq. (1) can be approximated by U = RI + LdI/dt (2) As follows from Eq. (2), the current I rises progressively until the time ti at which it reaches a certain value Ia (hereafter referred to as actuating value) in accordance with the following equation: I = U/R(1 - exp(- R/Lt)) (3) The electromagnetic force corresponding to the time t1 has a value sufficient for the armature to overcome the friction forces and start its movement from one operating position towards another operating position. At the stage, when the armature moves the gap between the armature and stator core changes.

In its turn, the changes of the gap courses a change of the inductance L of the electromagnet. Thus, Eq. (1) at this stage can be presented by <BR> <BR> <BR> U = RI + L dl + I dL (4)<BR> <BR> <BR> dt dt

The term I dL/dt in Eq.(4) is equivalent to the appearance of additional electric resistance in the circuit (1 in Fig. 1), that results in the drop of the current I, and respectively, the drop of the tractive force of the electromagnet. When the armature attains another operating position at the time t2, the inductance L of the electromagnet stops to change. Therefore, the current I changes its direction from the drop to increase, and reaches its final (steady-state) magnitude of U/R at the time t3.

The steady-state magnitude U/R of the current I may be large, which may result in the significant generation and dissipation of heat. The heat may cause damage of the coil and heavy power losses.

On the other hand, this large magnitude of the current is usually not required, since it often produces excessive tractive force. As can be clear to a man versed in the art, a large tractive force is required mostly at the initial stage of the armature motion, i. e., when the armature moves against the force caused by the load to be driven as well as friction, while a smaller force is sufficient to maintain the armature at the final position after completing the motion.

In addition to the heat generation and heavy power losses, the operation coil may store a substantial amount of energy during the rise of the current, which may become a disturbing factor at the time of turn-off. A further disadvantage of using a large current is related to the generation of a substantial delay when the magnet is turned off.

Various drive circuits are known in the art for controlling the application current in an operation coil in order to reduce the total power dissipation (see, for example, U. S. Pat. No. 4,180,026 to Schulzke et al, U. S. Pat. No. 4,327,394 to Harper, U. S. Pat. No. 4,511,945 to Nielson; U. S. Pat. No. 4,520,420 to Ariyoshi, et al ; U. S. Pat. No. 5,267,545 to Kitson ; U. S. Pat. No. 5,381,297 to Weber; U. S.

Pat. No. 5,471,360 to Ishikava et al ; U. S. Pat. No. 5,914,849 to Perreira; U. S. Pat.

No. 6,061,224 to Allen).

As a conventional practice in the operation of the electromagnet, the driver

circuits operate to energize the operating coils initially with a large current to "pull in"the armature and thereby move the electromagnet in the second position.

Thereafter, the driver circuits maintain a continuous or ripple current in the coil having a reduced magnitude necessary to hold the armature during the period required for holding the electromagnet in the second position.

Typically, the rise of the current is maintained until the full extent of the armature movement. Hence, only when the current in the coil has reached saturation, the current is permitted to drop to a value which is sufficient to hold the armature in the second position.

It is also known that as the armature starts to move, the air gap between the stator and armature diminishes, thus, the armature accelerates towards the second position. The second position may be reached with considerable velocity that may cause a bouncing action. Such a bounce may produce an impact noise of the electromagnet operation, due to the armature bouncing against the armature stop and may also cause the fatigue and deterioration of parts of the electromagnet.

Various control circuits are known in the art in order to minimize the problem of bounce by minimization of the acceleration of the armature.

For example, European patent application No EP 0 376 493 A1 describes a method in which high voltage is supplied in the initial stage in order to obtain a rapid rise in the current flowing in the coil for the purpose of rapid acceleration.

Thereafter, the coil is disconnected from the voltage supply, allowing the current in the coil to decay until the current reaches a value below the hold value. When the velocity of movement is reduced to a small value, the current is again increased to the hold value.

Another example of the control circuits for controlling the velocity of an armature as the armature moves from a first position towards a second position is described in U. S. Pat. Nos 5,991,143 and 6,128,175 to Wright et al. The method includes selectively energizing the coil to permit the armature to move at a certain velocity towards the second position. A certain voltage corresponding to a

voltage across the coil is determined when the armature approaches the second position. This certain voltage is used as a feedback variable to control energy to the coil so as to control the velocity of the armature.

To summarize the discussion about conventional electromagnets, it is relevant to note that when an electromagnet has a long stroke, the current of excessive magnitude (i. e. higher than that required for moving the electromagnet from one operation position to another operation position) is maintained in the coil long time. There is, accordingly, a need in the art to provide a new method for driving an electromagnet that may eliminate the aforementioned disadvantages associated with the excessive magnitude of the current.

As far as electromagnetic motors are concerned, a basic scheme of a conventional process for driving an electric motor is illustrated in Fig. 3. This figure shows the operation of a simple DC motor 10 utilizing a single coil 11. A voltage DC source 12 provides voltage through the coil via sliding contacts 13 (or brushes) that are connected to the DC source 12. The sliding contacts 13 are arranged at the end of coil wires 14 and make a temporary electrical connection of the coil 11 with the voltage source 12. In the motor 10, the sliding contacts 13 can make a connection and flow of current through the coil wires every 180 degrees.

In the 0 degrees diagram, the sliding contacts 13 are in contact with the voltage source 12 and the current is flowing. The current that flows through a coil segment C-D interacts with the magnetic field H that is present, and the result is an upward force Fup on the segment. The current that flows through a coil segment A-B has the same interaction, but the force Fdm, vn acting on the segment is oriented in the downward direction. Both forces are of equal magnitude, but in opposing directions, since the direction of current flow in the segments is reversed with respect to the magnetic field. These forces produce a torque spinning the coil 11.

In the 180 degree diagram, the same phenomenon occurs, but the segment A-B is forced up and the segment C-D is forced down. In the 90 and 270 degree

diagrams, the sliding contacts 13 are not in contact with the voltage source 12 and no force (torque) is produced. In these two positions, the rotational kinetic energy of the motor keeps it spinning until the sliding contacts 13 regain contact.

Referring to Fig. 4a, a typical torque curve produced by the single coil DC motor 10 is illustrated. One drawback of this motor is the large amount of torque ripple produced by this motor. The reason for this excessive ripple is associated with the fact that the coil has a force pushing on it only at the 90 and 270 degree positions. The rest of the time the coil spins owing to its own inertia and the torque drops to zero.

Referring to Fig. 4b, the torque curve produced by a two-coil DC motor is illustrated. As the second coil is added to the motor 10, the torque curve is smoothed out. the resulting torque curve never reaches the zero point, thus, the average torque for the motor is greatly increased. It should be understood that as more and more coils are added, the torque curve approaches a straight line (TMAx) and may have very little torque ripple, and the motor may run much more smoothly.

Another conventional method of increasing the torque and rotational speed of the motor is to increase the current supplied to the coils. This is accomplished by increasing the permanent voltage that is supplied to the motor, thus increasing the current at the same time, since the input steady state current is approximately proportional to the input voltage. However, the large current in the motor's windings results in the significant generation and dissipation of heat, which wastes power and reduces efficiency of the motor.

An additional disadvantage of the aforementioned motors when they are powered by a battery (instead of a power line), is that the motor speed and torque output tend to be reduced as the battery becomes discharged. For example, a DC fan motor utilized for cooling a computer may reduce a rotation speed from 3500 rotations per minute to about 1000 rotations per minute, as the voltage from a battery would vary from about 28 V in a float state to about 21 V in a discharged state. Such a large variation in the rotation speed may result in the inadequate

cooling of a computer.

Various pulse width modulation (PWM) motor control techniques are known in the art that partially eliminate these problems (see, for example, U. S.

Pat. No. 3,213,343 to Sheheen ; U. S. Pat. No. 3,875,486 to Barton; U. S. Pat. U. S.

Pat. No. 4, 143,307 to Hansen; No. 4,150,324 to Naito; U. S. Pat. No. 4,153,864 to Minakuchi; U. S. Pat. No. 4,356,438 to Iwasaki; U. S. Pat. No. 4,720,663 to Welch et al. ; U. S. Pat. No. 4,910,447 to Masters; U. S. Pat. No. 5,077,515 to Arnauld ; U. S. Pat. No. 5,388,176 to Dykstra et al. ; U. S. Pat. No. 5,590,237 to Audemard ; U. S. Pat. No. 5,907,226 to Ohsawa et al.; U. S. Pat. No. 5,955,851 to Solie et al.).

In order to change the speed of the motor, these techniques utilize sequences of unidirectional voltage drive pulses having various parameters. In particular, the motor speed may be adjusted by the variation of pulse width and/or pulse frequency (the number of pulses per time unit).

Referring to Fig. 5, an example of voltage and current waveforms is illustrated in a typical conventional PWM electromagnetic motor, when the motor is driven by two sequences of unidirectional voltage drive pulses 51 and 52. At the time interval between to and tl, the supply voltage in the form of the sequence of the pulses 51 is applied to the operation coil of the motor. The current rises rapidly until it slightly exceeds its rated value Irl at tl. At the stage when the voltage is disconnected from the coil, the current drops until the value is slightly below Irl at t2. The cycle is repeated throughout the sequence of the pulses 51, thus providing a ripple current in the coil maintained near its rated value IrlX Since the deviations of the amplitude of ripple current from the value Irl are relatively small, such a ripple current hereinafter will be referred to as a quasi-steady-state current.

At the time interval between ti and ti+j, the sequence of the longer duration pulses 52 of the supply voltage is applied to the operation coil of the motor. The operation cycle of the motor is carried out throughout the sequence of the pulses 52, thus providing a quasi-steady-state current in the coil maintained near its rated value 1, 2. As can be appreciated, the longer the pulses, the higher the current

in the coil (i. e. I,. > I,. l), and therefore, the higher the toque and faster the motor turns, and vice versa. Thus, controlling the width or frequency of the voltage pulses determines the average value of the current in the coil of the motor and hence, its torque and velocity.

One of the drawbacks of the PWM motors is that the current values Irl and Ir2 in the motor's windings are relatively large and may produce the generation and dissipation of excessive heat, which wastes power and reduces efficiency of the motor. Thus, as well as in the case of electromagnets, the problem of excessive magnitudes of the current maintained in the coil of the conventional motors still exist.

Thus, despite the extensive prior art in the area of driving electromagnets and motors and controlling the current maintained in the coils, there is still a need in the art for further improvements for driving electromagnetic devices.

SUMMARY OF THE INVENTION The present invention satisfies the aforementioned need by providing a novel method and system for driving an electromagnetic device and controlling physical characteristics thereof. Examples of suitable electromagnetic devices include, but are not limited to, electromagnets, actuators, relays and electric motors.

The drive of the electromagnetic device is carried out by feeding the device with a train of drive voltage pulses. In contrast to the conventional way of operating the device in a continuous duty cycle operation by a nominal operating voltage, the drive voltage pulses, according to the invention, have amplitudes which are significantly higher than the amplitude of the nominal operating voltage. A duty cycle of the drive voltage pulses (a ratio between the time signal in a high state and period of cycle) is chosen to assure that an amplitude of electric current in an operating coil of the device maintained in the time interval between any two drive voltage successive pulses, drops to a predetermined value that is substantially smaller than the maximum current magnitude of each corresponding current pulse.

By using such a scenario of feeding the device, the energy of an electric power source is redistributed between the energy transformed into heat and mechanical work for operation of the electromagnetic device. The redistribution is such that a ratio of the mechanical work to the energy transformed into heat is higher than the corresponding ratio achieved with the operation of said electromagnetic device in the steady-state operation regime, e. g. continuous duty cycle operation regime.

In order to avoid overheating of the electromagnetic device, an average current value over the time period of each drive voltage pulse should not exceed an average current maintained in the coil in a continuous duty cycle operation of the device.

According to the invention, the width of each drive voltage pulse occupies a time that is very short when compared to characteristic time constants of said electromagnetic device. An example of a characteristic time constant includes, but is not limited to, a ratio between the inductance L and resistance R of the operating coil of the device, to wit, L/R.

When a method of the present invention is utilized for driving an electromagnet, the train of the drive voltage pulses includes at least two pulses applied for urging an armature of the electromagnet to move from a first operating position towards and land at a second operating position.

In order to avoid a bounce of the armature from the second operating position, at least one additional drive voltage pulse may be supplied at the time when the armature lands at the second operating position.

After the armature arrives to the second operating position, the drive voltage pulses are adjusted so as to generate an electromagnetic pull sufficient for holding the armature at this position. Examples of the adjustment include, but are not limited to, decreasing the amplitude and/or width of the drive voltage pulses.

When the method of the present invention is utilized for driving an electric motor, the shape and amplitude of the voltage pulses depend on the type of the motor.

Thus, if the electric motor is a DC battery-powered motor then, for example, identical rectangular-shaped unipolar pulses having a predetermined amplitude and duty cycle may be utilized. If an AC utility power line is available, then the DC motor may, for example, be fed by short unipolar voltage pulses modulated by a sine wave corresponding to the AC power line. According to one example, a frequency of the sine wave may be defined by the frequency of the AC power line.

According to another example, the frequency of the sine wave may be two times the frequency of the AC power line.

When the method of the present invention is utilized for driving a multi-phase AC motor, the driving voltage pulses may be bipolar and have a rectangular shape for each phase. The amplitude of the driving pulses may be modulated by a sine wave of the AC power line for each phase. A frequency of the sine wave may be defined by the frequency of the AC power line or be two times the frequency of the AC power line.

The foregoing need is also accomplished by providing a driving system for an electromagnetic device. The system enables to decrease the amount of heat generated by the electromagnetic device by the controlled redistribution of the energy of the power source between the energy transformed into heat and mechanical work for operation of said electromagnetic device. Thus, when compared with the operation of the electromagnetic device in a continuous duty cycle operation, a smaller portion of the energy is transformed into heat and most of the energy is transformed into the mechanical work.

According to the invention, the system for driving an electromagnetic device by a train of drive voltage pulses includes a voltage source, a switch, sensing means and a controller unit. The switch is coupled to the voltage source and to the sensing means. In turn, the sensing means are coupled to the electromagnetic device and to a controller unit. Thereafter, the controller unit is operatively coupled to the voltage source, the switch and the sensing means.

The voltage source is controllable by a voltage control signal and configured for providing an energizing voltage of a predetermined magnitude. The switch is

controllable by a switch control signal and configured to supply the drive voltage pulses, which are obtained by chopping the energizing voltage, to the electromagnetic device.

According to a preferred embodiment of the present invention, the sensing means include a current sensor, a voltage sensor, a speed sensor, a pulse width sensor, and a pulse frequency sensor.

The current sensor is coupled to the controller unit and the switch. It is also connected in series with the electromagnetic device for coupling the switch thereto.

The current sensor is operable for producing a current sensor signal representative of an electric current in an operating coil of the electromagnetic device.

The voltage sensor is coupled to the electromagnetic device and to the controller unit. The voltage sensor is operable for producing a voltage sensor signal representative of the voltage amplitude of said drive voltage pulses.

The speed sensor is coupled to the said electromagnetic device and to the controller unit. The speed sensor is operable for producing a speed sensor signal representative of a speed of the armature.

The pulse width sensor is coupled to the electromagnetic device and to the controller unit. The pulse width sensor is operable for producing a pulse width sensor signal representative of a width of said drive voltage pulses.

The pulse frequency sensor is coupled to the electromagnetic device and to the controller unit. The pulse frequency sensor is operable for producing a pulse frequency sensor signal representative of a frequency of said drive voltage pulses.

The controller unit is responsive to the aforementioned sensor signals and configured for controlling the operation of the system by generating the switch control signal and the voltage control signal which are utilized as feedback variables.

Thus, according to one broad aspect of the present invention, there is provided a method for driving an electromagnetic device comprising energizing the electromagnetic device by applying a train of drive voltage pulses across an

operating coil of the electromagnetic device so as to produce a sequence of transient current pulses therethrough each having respective current amplitudes ; the method characterized in that: said drive voltage pulses have amplitudes significantly higher than the amplitude of a nominal operating voltage capable of operating said electromagnetic device in a continuous duty cycle operation regime; and a duty cycle of said drive voltage pulses is chosen to assure that the current amplitude drops in the time interval between any two successive drive voltage pulses to a predetermined value that is significantly smaller than a maximum current amplitude of each current pulse; whereby energy supplied to said electromagnetic device by an electric power source is redistributed between a first energy component that is transformed into heat and a second energy component that is transformed into mechanical work for operation of said electromagnetic device, such that a ratio of the second energy component to the first energy component is higher than a corresponding ratio achieved with operation of said electromagnetic device in the continuous duty cycle operation regime.

In accordance with another broad aspect of the invention, there is provided a system for driving an electromagnetic device having an operation coil and an armature by a train of drive voltage pulses, said system comprising: (a) a voltage source controllable by a voltage control signal configured for providing an energizing voltage of a predetermined magnitude; (b) a switch coupled to said voltage source and controllable by a switch control signal, the switch being operable to supply said drive voltage pulses by chopping said energizing voltage to said electromagnetic device; (c) sensing means coupled to said electromagnetic device, and being configured for producing at least one sensor signal from the following list:

a current sensor signal representative of an electric current in the operating coil, a voltage sensor signal representative of the voltage amplitude of said drive voltage pulses, a speed sensor signal representative of a speed of the armature, a pulse width sensor signal representative of a width of said drive voltage pulses, a pulse frequency sensor signal representative of a frequency of said drive voltage pulses, and (d) a controller unit operatively coupled to said voltage source, said switch, and said sensing means; said controller unit being configured for controlling operation of the system by generating at least one of said switch control signal and said voltage control signal; said controller unit being responsive to said at least one sensor signal; the system being configured such that: said drive voltage pulses have amplitudes significantly higher than the amplitude of a nominal operating voltage capable of operating said electromagnetic device in a continuous duty cycle operation regime; a duty cycle of said drive voltage pulses is chosen to assure that the current amplitude in the operating coil in the time interval between any two successive drive voltage pulses drops to a predetermined value that is significantly smaller than the maximum current magnitude of each pulse, whereby energy supplied by said voltage source means is redistributed between a first component of the energy transformed into heat and a second component of the energy transformed into mechanical work for operation of said electromagnetic device, such that a ratio of the second component to the first component is higher than a corresponding ratio achieved with operation of said electromagnetic device in the continuous duty cycle operation regime.

There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows hereinafter may be better understood. Additional details and advantages of the invention will be set forth in the detailed description, and in part will be appreciated from the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a simplified electric circuit of a typical electromagnet.

Fig. 2 is a schematic diagram illustrating voltage and current waveforms of a typical electromagnet.

Fig. 3 is a basic scheme of a conventional process for driving an electric motor.

Fig. 4a is a typical torque curve produced by a single coil DC motor.

Fig. 4b is a typical torque curve produced by a two-coil DC motor.

Fig. 5 shows an example of voltage and current waveforms in a typical conventional PWM electromagnetic motor.

Fig. 6 is an example of voltage and current waveforms for driving the electromagnetic device.

Fig. 7 is another example of voltage and current waveforms for driving the electromagnetic device.

Fig. 8 is yet another example of voltage and current waveforms for driving the electromagnetic device.

Fig. 9 shows a redistribution of energy supplied by an electric power source when the electromagnetic device is operated by steady-state currents and a way of operating the electromagnetic device by the transient current pulses, according to the present invention.

Fig. 10 is an example of voltage and current waveforms for driving an electromagnet, according to one non-limiting example of the invention.

Fig. 11 is an example of voltage and current waveforms for driving a DC electric motor.

Fig. 12 is an example of voltage and current waveforms wherein a train of the unipolar drive voltage pulses is modulated by a frequency of the AC power line.

Fig. 13 is another example of voltage and current waveforms wherein a train of the unipolar drive voltage pulses is modulated by a frequency of the AC power line.

Fig. 14 is a schematic block diagram of a system for driving an electromagnetic device, according to one embodiment of the present invention.

Fig. 15 is a schematic block diagram of a system for driving an electromagnetic device, according to another embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS The principles and operation of a method and a system according to the present invention may be better understood with reference to the drawings and the accompanying description, it being understood that these drawings are given for illustrative purposes only and are not meant to be limiting.

Referring to Fig. 6 through Fig. 8 together, various examples of voltage and current waveforms for driving an electromagnetic device are illustrated. It should be understood that these examples and the examples that follow hereafter are not intended to be limiting in any way.

It should be appreciated that the scenario of driving an electromagnetic device, according to the present invention, differs from the conventional way of driving the device. As was described above, conventionally, the electromagnetic device is fed with either a permanent nominal operating voltage or with relatively long voltage pulses so that the device mainly operates over the time intervals of steady-state or quasi-steady-state current conditions.

It should be appreciated that according to the present invention, the operation of the electromagnetic device occurs during the state of transient current in the operating coil following a coil disturbance by an energizing voltage. The transient current in the form of transient current pulses Pi is produced when a train of drive voltage pulses Pu is applied across the operation coil of the electromagnetic device. The drive voltage pulses Pu are characterized in that their amplitudes have high values and their pulses are relatively short.

For example, in contrast to the conventional continuous duty cycle operation regime (wherein the device is fed with a permanent nominal operating voltage), the drive voltage pulses Pu have amplitudes U, which are significantly higher than the amplitude of the nominal operating voltage Unom.

In practice, the amplitudes U of the drive voltage pulses Pu may, for example, be defined by a breakdown voltage between the windings of the operating coil. Depending on the electromagnetic device, the amplitudes U may vary in a broad range. According to one example, amplitudes U of the drive voltage pulses Pu may be 3 to 100 times the nominal operating voltage.

According to another example, the amplitudes U may be at least two times larger than an entire voltage drop on the operating coil.

According to the invention, a duty cycle of the drive voltage pulses (i. e., a ratio between time signal in high state and period of cycle) is chosen to assure that the current amplitude of the corresponding current pulses PI in the time interval At between any two drive voltage successive pulses drops to a predetermined value IMIN that is significantly smaller than the maximum current magnitude ImAx of each current pulse.

According to one embodiment of the invention, the predetermined value of the current IMIN should not exceed the value To of the current (see Fig. 6) defined by <BR> <BR> <BR> <BR> 1 nom<BR> <BR> <BR> UlUnom (S)

wherein I..... is an average current amplitude in the continuous duty cycle operating regime, U is the amplitudes of the drive voltage pulses Pu, and Unom is the amplitude of the nominal operating voltage.

According to another embodiment of the invention, the predetermined value of the current IMIN should not exceed 5% of the maximum current magnitude IA « X.

According to yet another embodiment of the invention, the current amplitude of the current pulses PI in the time interval At drops to zero (i. e., IMIN = 0). Fig. 7 illustrates an example of the situation when IMIFO In this example, the current amplitude corresponding to a second pulse of the two successive pulses starts to rise immediately after the current decay time ##d, i. e., the time required for the current amplitude corresponding to a first pulse to reach zero value. Fig. 8 illustrates another example of the situation when IMIN=0-According to this example, the current amplitude corresponding to a second pulse of the two successive pulses starts to rise only after time period Ar passes after the current decay time Aid.

In order to avoid maintaining in the operating coil a current having a high value, the width of the pulses Pu is chosen to be short. Preferably, the width of each drive voltage pulse should occupy a time that is very short when compared to the characteristic time constants of the electromagnetic device. An example of a characteristic time constant includes, but is not limited to, a ratio between the inductance L and resistance R of the operating coil of the device, to wit L/R. In such a case, a relationship between the amplitudes U of each drive voltage pulse, the maximum current magnitude IA (amplitude of each pulse), a width of the voltage pulse tU and an inductance of the operating coil L can be presented by: UtU IMAX = (6) L In order to avoid overheating the operating coil, an average current value 14v over the time period, T = tU + At, of each drive voltage pulse in the operating coil should not exceed an average current maintaining in the coil in the

steady-state or quasi-steady-state regime, e. g., in the continuous duty cycle operation regime.

As was described above (see Eq. (1)), when a voltage pulse is applied across the operating coil, the voltage drop on the coil includes an active term (depending on the coil resistance R) and a reactive term (depending on the coil inductance L and the current rate d/dt I). It should also be appreciated that whendt the amplitudes U of the drive voltage pulses PU are high and the width of the pulses is short, the rate of the current rate is also high. Under these circumstances, according to the invention, an active term of a voltage drop on the operating coil is substantially smaller than a reactive term of said voltage drop.

Referring to Fig. 9, a redistribution of energy 91 supplied by an electric power source is illustrated when the electromagnetic device is operated in a conventional way 10 (e. g., by steady-state currents) and in a way 20 of operating the electromagnetic device by the transient current pulses PI (produced by the drive voltage pulses Pu), according to the present invention.

As may be appreciated by a man versed in the art, when the electromagnetic device is operated by the transient current pulses, the energy 91 supplied by the electric power source is redistributed between a first component of the energy 92 transformed into heat and a second component of the energy 93 transformed into mechanical work for operation of the electromagnetic device. The redistribution is such that a ratio of the second component to the first component is higher than the corresponding ratio achieved with the operation of said electromagnetic device in the steady-state operation regime, (e. g. continuous duty cycle operation regime). In other words, when compared with the operation of the electromagnetic device in a continuous duty cycle operation, a smaller portion of the energy is transformed into heat, and most of the energy is transformed into mechanical work.

According to the invention, examples of suitable electromagnetic devices are electromagnets, electric motors, actuators, relays, etc.

Referring to Fig. 10, an example of voltage and current waveforms is illustrated for driving an electromagnet, according to one non-limiting example of the invention.

According to this example, the train of the drive voltage pulses Pu includes two rectangular pulses 1001 and 1002 which are applied across the coil for pulling an armature of the electromagnet to move from a first operating position towards and land at a second operating position.

When the armature arrives and lands at the second operating position, an additional rectangular drive voltage pulse 1003 is applied at the time tland. This pulse is applied in order to retain the armature at the second operating position and avoid a bounce of the armature.

After the armature lands at the second operating position, the drive voltage pulses Pu are adjusted (see, e. g., a sequence of holding rectangular pulses 1004) so as to generate an electromagnetic pull sufficient for holding the armature in this position. Examples of the adjustment include, but are not limited to, decreasing the amplitude and/or width of the drive voltage pulses.

The train of the current pulses PI in the operating coil includes current pulses 2001,2002,203 and 2004 corresponding to the voltage pulses 1001,1002, 1003 and 1004, respectively.

This scenario was utilized for driving an electromagnet having the following parameters: Electromagnet Mass-0.6 kgm; Stroke Length-10 mm ; Coil Inductance-30 milli Henry; Coil Resistance-22 Ohm; Windings Number-1000; Nominal Voltage-48V; Nominal Power-300 W The following characteristics of the equal drive voltage pulses 1001,1002 and 1003 were set:

Voltage Pulse Amplitude, U-300 V; Voltage Pulse Width, tu-2 milli sec; Voltage Pulse Period, tu +At 10 milli sec; Voltage Pulse Duty Cycle-20 % The following magnitudes of the current in the operating coil of the electromagnet were measured: Current Amplitude of the Pulse 2001-3A; Current Amplitude of the Pulse 2002-6A; Current Amplitude of the Pulse 2003-12A; Current Amplitude of the Pulses 2004-0.2-0.3 A The measured stroke time tlandn (i. e., the time required for the armature to move from the first operating position to the second operation position until it lands thereon) was about 16 milli sec.

When the method of the present invention is utilized for driving an electric motor, the shape and amplitude of the voltage pulses depend on the type of motor.

Referring to Fig. 11, an example of voltage and current waveforms is illustrated for driving a DC electric motor. According to this example, a train of identical rectangular-shaped unipolar pulses Pu having a predetermined amplitude and duty cycle are applied across the coil for spinning the motor having the following parameters: Coil Inductance-2.2 milli Henry; Coil Resistance-1 Ohm; Nominal Voltage-24V; Nominal Power-100 W The following characteristics of the equal drive voltage pulses Pu were set: Voltage Pulse Amplitude, U-300 V ; Voltage Pulse Width, tu-170 micro sec; Voltage Pulse Period, tu +At-1 milli sec ; Voltage Pulse Duty Cycle-17 %

The following characteristics of the train of the current pulses Pi in the operating coil of the electric motor were measured: Maximum current magnitude, Iz4x-10 A; Current decay time AId ~ 60 micro sec; The average current estimated over the time tu +ATd corresponding to the current pulse was about 5 A.

The average power of the electric motor estimated over the time pulse period estimated as a product of the duty cycle, voltage pulse amplitude and average current was 255 W. As may be appreciated, this value of the power obtained as a result of utilizing the train of the voltage pulses for driving the motor is about 2.5 times higher than the nominal power of the motor.

As may be appreciated, a DC motor may be fed with either identical drive voltage pulses or by the voltage pulses modulated by a sine wave. For example, if an AC utility power line is available, then a DC motor may be fed by drive voltage pulses Pu modulated by a sine wave corresponding to the AC power line. It is preferable that the frequency of the sine wave be at least four times smaller than the frequency of said drive voltage pulses. In other words, the pulse generation is such that at least two drive voltage pulses occupy the time interval corresponding to the half period of the sine wave.

Referring to Fig. 12, an example of voltage and current waveforms is illustrated, wherein a train of the unipolar drive voltage pulses Pu is modulated by a frequency of the sine wave of the AC power line. In this case, the frequency of the sine wave is twice the frequency of the AC power line.

When the method of the present invention is utilized for driving a multi-phase AC motor, the driving voltage pulses Pu may, for example, be bipolar and have a rectangular shape for each phase (see Fig. 13). The amplitude of the driving pulses may be modulated by a sine wave for each phase, for example, by the sine wave of the AC power line. In such a case, the frequency of the sine wave may be defined by the frequency of the AC power line or be two times the frequency of the AC power line.

Referring now to Fig 14, there is illustrated a schematic block diagram of a system 100 for driving an electromagnetic device 101 by applying thereto a train of drive voltage pulses Pu, so as to produce a sequence of current pulses TI therein, according to one embodiment of the present invention. It should be noted that the blocks in Fig. 14 are intended as functional entities only, such that the functional relationships between the entities are shown, rather than any physical connections and/or physical relationships.

The system 100 includes a voltage source 102, a switch 103, sensing means 110 and a controller unit 109. The switch 103 is coupled to the voltage source 102 and to the sensing means 110. In turn, the sensing means 110 are coupled to the electromagnetic device 101 and to a controller unit 109. Thereafter, the controller unit 109 is operatively coupled to the voltage source 102, the switch 103 and the sensing means 110.

It should be appreciated that examples of the electromagnetic device 101 that is driven by the system 100 include, but are not limited to, electromagnets, electric motors, etc.

The voltage source 102 is controllable by a voltage control signal Cu and configured for providing an energizing voltage of a predetermined magnitude U.

The switch 103 is controllable by a switch control signal Csw and configured to supply the drive voltage pulses Pu (which are obtained by chopping the energizing voltage) to the electromagnetic device 101.

According to a preferred embodiment of the present invention, the sensing means 110 include a current sensor 104, a voltage sensor 105, a speed sensor 106, a pulse width sensor 107, and a pulse frequency sensor 108.

The current sensor 104 is coupled to the controller unit 109, to the switch 103 and is connected in series with the electromagnetic device 101 for coupling the switch thereto. The current sensor 104 is operable for producing a current sensor signal Si representative of an electric current in an operating coil L of the electromagnetic device 101.

The voltage sensor 105 is coupled to the electromagnetic device 101 and to the controller unit 109. The voltage sensor is operable for producing a voltage sensor signal Su representative of the voltage amplitude U of the drive voltage pulses Pu.

The speed sensor 106 is coupled to the said electromagnetic device 101 and to the controller unit 109. The speed sensor 106 is operable for producing a speed sensor signal S representative of a speed of the armature A.

The pulse width sensor 107 is coupled to the electromagnetic device 101 and to the controller unit 109. The pulse width sensor 107 is operable for producing a pulse width sensor signal Stu representative of a width of said drive voltage pulses PU.

The pulse frequency sensor 108 is coupled to the electromagnetic device 101 and to the controller unit 109. The pulse frequency sensor 108 is operable for producing a pulse frequency sensor signal SF representative of a frequency of said drive voltage pulses Pu.

The controller unit 109 is responsive to the aforementioned sensor signals and configured for controlling operation of the system 100 by generating the switch control signal Csw and the voltage control signal Cu, which are utilized as feedback variables.

As may be appreciated, the electromagnetic device 101 may be fed with either identical voltage pulses or by the voltage pulses modulated by an external wave.

Fig. 15 illustrates an embodiment of the system (100 in Fig. 14) for driving the electromagnetic device 101 in which the system further includes an amplitude modulator 120 coupled to the controller unit 109 for amplitude modulation of the drive voltage pulses Pu by an external AC sine wave.

According to this embodiment, the voltage source 102 is coupled to an AC power line and to the amplitude modulator 120 that is synchronized with the AC power line.

According to this embodiment, the system may further include an AC rectifier 130 associated with the voltage source 102 and configured for rectifying the AC power line, and thereby providing a frequency of the sine wave to be equal to two times the frequency of the AC power line. It should be appreciated that the invention is not intended to be restricted to any particular voltage source/rectifier association. Hence, the rectifier 130 may be a constructional part of the voltage source 102 (as shown in Fig. 15) or a separate unit.

As such, those skilled in the art to which the present invention pertains, can appreciate that while the present invention has been described in terms of preferred embodiments, the concept upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, systems and processes for carrying out the several purposes of the present invention.

It is apparent that although the examples of drive voltage pulses Pu utilized for driving an electromagnetic device illustrate the utilization of rectangular shape pulses, the present invention is not intended to be restricted to any particular shape of the pulses.

Moreover, any reference to a specific implementation in terms of usage of the voltage source, switch, sensing means, control module, or any other components are also shown by way of a non-limiting example.

Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

It is important, therefore, that the scope of the invention is not construed as being limited by the illustrative embodiments set forth herein. Other variations are possible within the scope of the present invention as defined in the appended claims and their equivalents.